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What distance can you see aircraft lights from?
My parents bought a house and it's approximately 50-60 km from the nearest airport and one day when it was dark I've noticed lights on the sky and because I live in a big city close to the airport I came to the conclusion that this have to be airplane lights. Is it possible to see them from that far?

Sure, but how do you know the lights were 50-60km away? Airplanes fly all over the place, not just near airports...

50–60 km from airport is pretty close! The descent angle is around 3°, which means 1000 ft altitude per 3 NM or 50 m altitude per 1 km. From the typical cruise altitude, the aircraft starts to descend more than 100 NM/200 km from the airport and just the final approach is usually more than 10 km. So the aircraft may be passing at relatively low altitude (9—10,000 ft/~3 km at that distance) somewhere close by. With precise location it would be possible to find where the typical arrival and departure routes come closest.

Aircraft lights can be easily spotted 50-60 km away during clear nights.
I used to live in Rotterdam and could spot aircraft at night when they turned exactly towards me at ARTIP which is an initial approach fix of Amsterdam airport Schiphol. ARTIP was exactly 100 km from my house.

Aircraft lights can be seen from substantial distances. How far will depend primarily on two variables: visibility, and how bright the aircraft lights are.
When I am flying at night in good visibility, I can often see aircraft lights at great distances from my aircraft. I do not know how far the furthest are, but some of the nearer ones I have been able to establish at 30 NM (55 km), for example. I am confidant that some of the further ones are 50+ NM (92+ km). The aircraft in these examples would be aircraft with beacon, strobes, and nav lights on.
Now, in the same good visibility conditions, with visibility at 100-150 NM (180-270 km), I can often see city or town lights at 150 NM (270 km), and at 100 NM (180 km) may be able to positively identify that city or town. Cities give off much more light than typical aircraft lights, but aircraft lights against a dark sky may be visible from quite a distance. Unfortunately I cannot offer a definitive maximum distance from personal experience.
When viewed from the ground in good visibility conditions, there is a strong possibility that you could see aircraft in cruise at distances of 30-50 NM (55-92 km) or more. The brightest aircraft lights will typically be the landing lights, and since some aircraft extend or turn these on below 10000 ft, these may be visible when the aircraft is as far as 60 NM (110 km), or more, from the airport.
In your case, if you are positioned 50-60 km (27-32 NM) from the airport, you may very well see aircraft overflying you in the descent towards the airport. Depending on the aircraft's orientation, especially if the landing light is pointed in your direction, you may also be able to see them beyond the airport on the approach.


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Why do most propeller-powered planes not have an autothrottle?
Why does every kind of propeller-powered plane lack an auto-throttle?
No single-props no twin-props, not even turboprops like the Dash-8 Q400. To be honest, I don't know many turboprops, but if the Dash-8 doesn't have one, why shouldn't others? I mean, even a mini Cherokee 180 has an autopilot, whats so hard implementing an auto-throttle? And the Dash-8 is an airliner... without auto-throttle!
Thanks for your answers, I would really appreciate your answers.

Ask yourself how expensive an autothrottle is vs how much safety/convenience benefit there is and I think you'll answer your own question.  There are many airliners without autothrottle that have flown safely for decades, and autothrottles have directly and indirectly contributed to accidents as well.

Turboprops like Dash-8's are flown by "entry-level" pilots who are looking to move up to the bigger jets. Probably part of the reason is the same logic behind not handing your teenager the keys to your Ferrari, they need to learn hands-on with something not-so-complicated. Also, usually turboprops are short-haul regional carriers that don't really need the hands-off that the bigger jets do.

@nexus_2006 may you post some links to the accidents caused my autothrottle? Would be interested. Thanks

God, I would be happy just to get fuel injection.

The words "auto" and "flying" are not good words to use together.

Because each jet engine only has one lever; each piston engine has three.
Jet engine's throttle is pretty straight forward - move forward, more power. The computer automatically manages fuel flow to prevent surge, flameout and N1 overshoot.
On a piston engine you have three - a mixture lever, a propeller lever and a throttle lever. Granted, I don't think auto-mixture would be too hard to implement. Now, do I want a high-speed climb or high-power climb? And remember, the prop and throttle must be managed together. You cannot go beyond a certain power range with the props set for low rpm, otherwise you'll damage the engine.
It's like a car's automatic transmission, except it has one more dimension.
Power affects pitch
When you change power you must change the aircraft's pitch altogether. Autopilots on modern airlines have quite a number of vertical modes: ALT, V/S, G/S, VNAV, FLCH, TO/GA, CLB etc. None of those have the concept of gear ratio, which is required for propeller management.
Can you design an autopilot system such that it functions on propeller engines? Absolutely. But it is going to be complicated. Not only the usual operations, but the failure modes, have to be engineered and learnt by the pilots. How do you deal with an auto-mixture failure? RPM mismatch? Such training would add unnecessary cost to an otherwise simple aircraft operation. If your airplane does not have a variable pitch propeller, it is probably too simplistic for this kind of autopilot anyway.
Jets have another property which makes auto throttle a logical choice - the faster you fly, the more powerful the engines. Thus, on a jet, you would have to make continuous throttle adjustment if you change your airspeed. So an auto throttle would be helpful. Pistons do not behave like that.
Turboprops
One of the main reasons for choosing piston aircraft is - it is simple. It just fly. No fancy stuff. No lengthy preflight checks. No computers. I just want to get flying. This thinking also extends to turboprops.
It's worthy to note that the ATR does not have auto throttle as well. The ATR 42 has power management (but not speed management), where activating it will simply move a bug on the torque gauge; the pilots have to manually move the levers to follow it. The ATR 72 has a notch for the pilots to place the throttle levers, but still, no motor to move the levers automatically. I can't think of any reason behind this design philosophy besides simplicity and cost.

It's also a major safety factor on jets at commercial cruise altitude where coffin corner is small.  Leaving it to the pilot would be problematic on long cruises.

Some aircraft diesels have FADEC and a single power lever, so in theory wouldn't autothrottle be as simple for them as for a jet?

Autothrottles are a wonderful piece of automation to have, but they are expensive to install and maintain, and the technology behind them is complicated and can be easily misunderstood by a pilot who isn't fully up-to-speed with the automation and "what's it doing now."
An autopilot is pretty easy to grasp, conceptually.  Turn on altitude hold, it does.  Turn on heading hold, it does.  Couple the autopilot to a VOR or GPS course, and it holds that.  There's no energy management, no interactions between the left/right and the airspeed.  Navigating vertically changes that, because you have a lot of coupling between the vertical modes (i.e. things driving the elevator) and the throttle modes (i.e. things driving the throttle).  This translates into more complicated software, more controls, and more required pilot understanding of what's going on in the FMC and autopilot.
Beyond this, when you have an autothrottle, you need to make sure that it will disengage fully when you tell it to, that it can be overridden with pilot input, and that it won't drive the engine into an out-of-limits condition (RPM, Manifold Pressure, Torque, EGT, etc).  Jet engines are generally pretty simple in this regard (particularly when they're already protected with a FADEC) compared to turboprop or (especially) reciprocating engines.  Do you really want your autothrottle trying to lean out the engine in the climb?
The end result of all of this is the the economics tend not to be worth it to put autothrottles in the various prop-driven aircraft.

Are turboprops really that much more complex than a jet?

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Which procedures or devices do ensure the aircraft stop flying after the pilot has ejected?
Pilots eject from aircraft when it cannot be controlled any more. Ejection seems to happen without delay and few time to secure the aircraft and its weapons.

Pilot ejection, source.
Thanks to @RonBeyer for pointing out the Cornfield Bomber which also links to a notable accident in 1989 with a MiG-23 flying 900 km and several countries, uncontrolled:

During takeoff [from Poland], the afterburner failed and the engine
  began losing power. At an altitude of 150 meters and descending, the
  pilot assumed he had a complete engine failure and ejected. [...]
  After flying over 900 km (560 mi) the MiG crashed into a house,
  killing a Belgian teenager.

What are the procedures or features in place to prevent the aircraft to keep flying and explosives to explode?

Related: Cornfield Bomber

I don't believe there is a mechanism for this. The aircraft will crash (and possibly explode), the ordinance typically will not (shouldn't be armed until after launch), which is why a lot of ejecting pilots are commended with their ability to "direct the aircraft away from people" before ejecting.

Is it realistic to imagine the aircraft continuing to fly without pilots? The pilot is generally ejecting because the aircraft is no longer able to fly with a pilot, how could it then fly without?

Devices? Does the ground count?

I did say generally :) So the question is about the scenario where a pilot ejects unnecessarily. I'd hope this is extremely rare though as that incident proves, it is possible. I think the thought of ejection and having to explain millions of dollars going up in flames to your boss will keep it a rare event.

@Notts90: You could say the same for many things on aviation, it should be rare, but we add security nonetheless, e.g. to prevent raising landing gear on the ground. What is the cost of the safety device compared to the cost of damages? In addition here we are talking about (civilian) lives.

@mins I agree that the attention to edge cases is what gives aviation such good safety records. However the aircraft will stop flying eventually, even if from lack of fuel. I think the far more important issue is where it stops.

This archived page has pictures of the f-106 after its landing in Montana.https://web.archive.org/web/20110106092409/http://www.f-106deltadart.com/71fis_PilotlessLanding_580787.htm

That's an impressive feat for an 1898-era aircraft, with or without pilot.

@vasin1987: Before Ron Beyer's edit, the post claimed that the MiG incident took place in 1898.

As you pointed out, the only reason for abandoning an aircraft is because the pilot can no longer control it or successfully land it.  With or without flight crew onboard, it's going down in one form or another.  As such it's pointless to design any additional system to disable or destroy the aircraft as it accomplishes nothing.
Pilots however have been known to direct a stricken or otherwise doomed aircraft away from populated areas, as this Thunderbirds pilot did at a 2003 air show at Mountain Home AFB, in Idaho.

So too was this crash of a MiG-29 at the Paris Airshow in 1989.  Mikoyan test pilot Victor Kobachev ingested a bird in his starboard engine and the jet departed controlled flight;  Kobachev fought to direct it away from the crowd line before he punched out.

@mins An aggressor just shot your wing off, how do you decide where to crash? Explode in the air and you're just raining shrapnel down over possibly miles of people, at least when it crashes it is relatively contained.

Now I have heard stories of pilots having in flight emergencies where the crew directed the aircraft away from a populated area prior to abandoning the airplane.

Here's the follow up to last year's crash by the Thunderbird after the AF Academy fly over.  Great video from the Denver Post.  Although the F-16 looks like it just landed in the field, it was totaled.

In September 16, 2005, a Russian jet Su-27 crashed in Lithuania after running out fuel (navigation error) (ref1, ref2).
I cannot find the online sources after such a long time but I remember the pilot said that he used the last remaining fuel to bring the plane away from any settlements, before ejecting. The jet, indeed, crashed in the fields with no notable danger to the people.
From this looks like the pilot is responsible for discarding the plane safely, as much as this is still possible. In cases like running out of fuel might be possible.

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How to read representations of pressure around an airfoil?
Actual representations found online
I selected 6 online representations of pressure around an airfoil, mostly using inconsistent symbolization of the pressure field.

Source of the individual pictures: Fig 1, fig 2, fig 3, fig 4, fig 5, fig 6
As visible, arrows can be found in nearly every combination of directions. On figure 4, pressures are about 2 hPa (200 Pa) and 5 hPa, which is not really understandable when pressure at sea level is about 1,000 hPa.
Last figure shows the pressure field in the cross-section, not only at the surface of the airfoil (which I believe is the case for the other figures).
Assumptions made to read the figures
By following the answers on this site, I know that the pressure on the lower side of the airfoil is greater than the pressure on the upper side. I also assume that the pressure on the upper side is lower than the ambient pressure, and the pressure on the lower side is greater than it. I believe the drawings are trying to depict these two aspects, but using different (and unspecified) conventions.
Probably the clearer convention is the one used on fig 6: Yellow is likely the ambient pressure, orange would be greater, green lower, blue even lower.
Questions

What are the usual pressures found around an airfoil, e.g. in cruise, at mid-span of a A320 wing (compared to the ambient pressure).
What is the most correct representation of the pressure field in the figures above?
Is there some conventional way used by engineers to depict the pressure field without ambiguity?

The arrows are more for the "gee wiz" factor - while they help to show in which direction which pressure is acting, the regular way of depicting pressures is using the pressure coefficient.
Now to your figures:

This probably tries to illustrate the fact that there is air pressure all around the airfoil, and less on top than on the bottom. But the length of the arrows does not correspond to any real pressure distribution. Please do not use this plot to learn aerodynamics.
This one tries to be more realistic, but gets the pressure distribution around the nose totally wrong. While better than 1, it is still not recommended.
This one is clearly the best of the bunch. Here you have a small but intense overpressure region at and near the stagnation point and suction on both sides, as would be the case at small and medium angle of attack if the airfoil has some thickness. The displacement effect will cause the flow on both sides to accelerate to a speed above flight speed, but camber and angle of attack make this effect much more pronounced on top. At the trailing edge you will find a small overpressure on top and a little suction at the bottom when the flow negotiates around the trailing edge because the boundary layer on the upper surface is much thicker than on the lower surface.
Here you see absolute pressure difference numbers, which is unusual. While the figures look realistic, this plot is also not really helpful because it omits the pressure distribution in the forward part of the airfoil where most of the action takes place. I think this is another special plot to illustrate one specific aspect. The numbers are the pressure difference to ambient pressure.
This one just tries to show how the pressure varies along one streamline. But the streamline has a constant distance from the airfoil, which is wrong, because it would normally come closer to the airfoil surface in proportion to the local speed increase. Also, the stagnation area is stretched out too far backwards.
This is a typical plot from a Euler or Navier-Stokes code where the space around the airfoil is panelled instead of only the surface as is customary for potential flow. The plot looks credible, but without a legend you do not learn much about the pressures around the airfoil. Your assumption about the colours is correct.

Please note that 1 tries to show absolute pressure while the others show the pressure difference to ambient.
To answer your question: Most of the plots suck and it would be a waste of time to "read" them because of their limited truthfulness. Maybe this would improve when you read the accompanying text as well, but it would be even better if all plots were like number 3.
What helps most when reading pressure plots is consistency - here all 6 plots fail. The proper way is to plot the pressure coefficient as explained in the linked answer. Only then can you gain some familiarity with the representation and make meaningful comparisons among different airfoils or flow conditions. Negative values of the pressure coefficient $c_p$ denote suction and positive ones overpressure, all relative to ambient static pressure.


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How many Inertial Reference Systems are on a commercial aircraft?
Nowadays all new airplanes have GPS on board so the use of IRS may lose  importance, still they must be fitted. How many of them are usually on a commercial aircraft? Does the regulation impose the number?

Not quite a dupe but you can find the answer here

I don't think there's any regulation that says they must be fitted @Ghilardi, it's more of the fact they are still needed. GPS isn't 100% reliable, the US military holds the keys and can turn the system off if desired.

@GdD Selective Availability has been discontinued for almost 20 years (since 2000). The new GPS Block IIIA launching this year don't have the capability anymore.

I would expect the regulations to impose requirements on the results (such as a given likelihood of a failure of a system within so-and-so many flight hours), not on how those results are achieved.

The answer is at least two. Although depending on how you view the system, either one, two or three can be a valid answer.
In the past, each IRS (Inertial Reference System) is itself a unit, and manufacturers would put multiple number of this unit in the cockpit. For example, the Boeing 737 has IRS L and IRS R. The pilots can power each unit on or off individually; they can also choose which unit goes to which display (captain side or copilot side). So in the Boeing 737, the answer to your question would be two.
Fast forward to the Boeing 777, the pilots only have one such unit: the ADIRU, or Air Data Inertial Reference Unit. The ADIRU by itself is a fault tolerance unit - within this unit, there are multiple gyroscopes and accelerometers. In the past, when multiple redundant units are provided to the pilots, there were instances where the pilots had trouble identifying the malfunctioning unit or switched to the wrong unit, and the plane crashed. Now there is only one unit, and the redundancy mechanism is auto-managed. So to answer your question for the Boeing 777, the answer could be either one or three.
It should be noted that even GPS has redundancy: there are usually two receivers: GPS L and GPS R. On newer airliners, all the data - GPS L, GPS R, ADIRU, VOR/DME etc. are combined to form a navigation system. Therefore one can say, "In the past there were 2 IRS, which consisted of gyros, on airliners. Nowadays we still have gyros, but we don't call them IRS because they form an integrated system with other elements".

One
the units are now combined Air Data Inertial Reference Units (ADIRU) Boeing or air data inertial reference system (ADIRS) Airbus.
Each unit has redundant inputs  3x Air Data, 2 or 3 x Inertial Reference. The Flight Management Computer takes ADIRU/ADIRS information and compares it with GPS (or Radio Navigation data) to work out dynamic calibration to the INS data.
You have to remember that GPS can be jammed and spoofed so it is important that the INS is fully self contained

'GPS can be jammed and spoofed' indeed, and in the case of spoofing, (eg. by terrorists), I wonder what would happen, in practice, if the set of INS coordinates all showed a significant and consistent deviation from that reported by the set of GPS?   Which do you believe?

Modern GPS systems use 5 or 6 satellites to not only generate a position but also generate a error measurement to that position. This is referred to as ANP or Actual Navigation Performance (Imagine it as the size of the blue  position circle on your IPAD) if this measured error exceeds a predetermined value called the RNP or the Required Navigation Performance an alert is generated.

GPS provides position (and then track and groundspeed). But it doesn't produce attitude, attitude rate or heading information, for which multiple sources are needed to provide redundancy and error-checking.
So there's 2 or 3 IRSs or equivalent. There would usually be multiple (2 or 3) GPSs, which might or might not feed into the IRSs, depending on the system architecture.


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How to advise others of imminent danger?
In a case such as United 326 - where an aircraft got cleared for takeoff while other planes at the same time get cleared to cross that runway - how would an outside party that picks up on the mistake inform the others that a disaster is about to happen and that people need to take immediate action?
Would they call mayday, or just shout out in "freeform", something along the lines of "Imminent collision, runway 19C!".

"Hey tower, did you mean to clear United 236 while clearing the crossing for Oceanic 123?"

@RonBeyer I meant in a case where things are seconds away from going disastrous, where people need to act immediately or there will much sadness.

If we're talking literally seconds, then keep in mind that the pilots involved will need to (1) realize that you're talking about them, or to them; (2) determine the action required by them to resolve the situation; (3) execute that action; and (4) have the action take effect. That's a mouthful, especially since even your brief transmission would take at least some 2-3 seconds to rattle off (at the standard rate of about 100 words per minute, and assuming you say "19C" as "nineteen center"; not that "nineteen see" or "one nine see" would save you much time...).

@MichaelK Usually seconds doesn't mean enough time to get a large airliner out of the way. A loaded 737 can weigh more than 70 metric tons and engines don't react as quickly as they do in the movies. Even if you started screaming "STOP STOP STOP" into the radio (which will get everybodies attention), a few seconds will just mean more time for the pilots to realize how bad this is going to be. An airliner goes from "cleared for take-off" to the end of the runway in less than 30-45 seconds.

After watching the video, I think the first real problem would be to cut in and say anything at all in this constant chatter!..

@Zeus If it's really as important as OP indicates, I wouldn't bother waiting to cut in; just transmit. A continuous "STOP STOP STOP STOP STOP STOP" similar to what Ron Beyer suggests doesn't need much time in between other transmissions to get through intelligibly. The harder part is for the person at the receiving end to identify who needs to stop -- does this apply to them? Braking hard is going to give the passengers a bit of a jolt; I dare say no pilot wants to give their passengers a rough ride.

@Michael, but you can't transmit (usefully) when someone else is transmitting, and in that case someone was always talking!

@Zeus True, but as soon as that other individual lets go of the button, your "OP STOP STOP STOP" gets through. Or, if you're lucky, your signal ends up being considerably stronger and overpowers the other transmitter sufficiently to make it through semi-intelligibly.

To quote an answer by kevin on another question from last year:

What's the procedure? The procedure is, be creative to save as many lives as possible!
Really. The procedure is to determine a course of action which will likely result in the best outcome for everyone, utilizing all resources and given all constraints. Period. It is as simple as that.
There are infinitely many scenarios, and one cannot be trained for everything.

If you need to communicate something as quickly as possible in order to avoid a disaster, and calling "mayday, mayday, mayday" will be a significant waste of valuable time, then the answer is clear: don't call mayday. Just say what you need to say.
If you know what the other aircraft need to do in order to avoid a collision, perhaps it would be a good idea to announce "Go around, go around, go around!", "Cancel takeoff, cancel takeoff, cancel takeoff!", or "Hold short, hold short, hold short!", as appropriate.
In the case of Air Canada 759, where an arriving airliner mistakenly lined up to land on an occupied taxiway instead of the runway, a pilot on the taxiway announced: "Where's this guy going? He's on the taxiway." This probably wasn't the best possible thing to say, but the airliner went around and avoided a collision.

I feel honored to be quoted (-:

So in brief you are saying: cut in, get the message across, procedures matter less than saving lives.

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Does the F-16 have any armor at all?
When I read about the A-10 I'm met with statements (e.g. Wikipedia) such as:

The A-10 is exceptionally tough, being able to survive direct hits from armor-piercing and high-explosive projectiles up to 23 mm. It has double-redundant hydraulic flight systems, and a mechanical system as a back up if hydraulics are lost.

What about the F-16 (from A to the latest block 70/72, maybe even F-21 should be included)? Will even small arms fire risk killing the pilot as well as destroying vital systems?
Although it was not designed as a ground attack aircraft, it seems that its role in ground support missions becomes more and more prevalent. The aircraft has also shifted its design goals from being as light and fast as possible to be heavier and feature more complex systems. The F-16's weight seems to only go up with each revision.
If the F-16 is supposed to loiter and provide ground support, it seems that some light (composite? Kevlar?) protection for at least the pilot from small arms fire and slightly heavier weapons would not be a bad idea. Keep in mind that I'm naive about this though, which is why I'm humbly asking.
From what little I know about the F-16, I believe it has quadruple redundant flight computers for its fly-by-wire system, and also has a redundant hydraulics systems (A and B, isolated by a check valve) which both can operate the FLCS (although beyond this, there are differences, A and B are not duplicates), and the EPU.
Design choices like this are obviously there to ensure the F-16 survives when systems fail. However, perhaps they are intended to ensure safe fail-over in terms of non-combat damage and failure.
I also understand that adding armor to a light and agile combat aircraft would be painful, as it adds weight and reduces flight time, makes the airframe thicker with perhaps slightly higher surface area/drag coefficient (?), and probably adds stress to an airframe that is supposed to be able to pull many G's in combat.
I've never heard of any armor whatsoever on the F-16. However, maybe there is? Is there any protection at all, to protect the pilot, or any of its critical systems (flight control computers?)? If no armor, perhaps the flight computers (of which I understand there are multiple physical ones), are they placed in different locations in the aircraft to make it less likely they are all lost from damage in the same location?
As a bonus question, this philosophy of no armor at all on fast combat aircraft, does this extend to most other aircraft? If there are a few notable exceptions, it would be nice to have a couple of the noteworthy ones mentioned (not looking for an exhaustive list).

See also: Are modern fighter jets bulletproof and armored?

No.  The aircraft in question do not have ballistic protection, though they do offer system redundancy for battle damage and have to meet certain survivability criteria in live fire tests during development.
Just a note:  the only armor an A-10 carries is a titanium pressure vessel for a cockpit which can take the kind of ballistic punishment you described.  It’s main defense is system redundancy, self sealing fuel tanks and engines mounted well away from the airframe and fuel.
Again the chief reasons combat aircraft do not carry external ballistic armor is any kind of armor over the exterior which can deflect the kind of AP/HE/Frag type warheads used in AAA/AAM/SAM warheads would make the aircraft prohibitively heavy and drastically reduce the warload and performance metrics the airplane needs to survive in combat.  And fighters like the F-16 and F-22 are considerably more vulnerable to enemy fire with engine surrounded by fuel tanks, causing even properly placed small arms fire to be catastrophic.

On the other hand, if you're armoured heavily enough to shrug off missile hits, you don't need spectacular performance to survive.

The A10 canopy is obviously not titanium and is part of the pressure vessel. Only the windscreen in the canopy is armored, rated for small arms fire. The titanium 'bathtub' is rated for 23mm high explosive and armor piercing cannon fire from most likely directions.

@Vikki said airplane would also be useless for a fighter, let alone even being able to fly.


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Why does the cockpit have windows, is it a structural design burden?
Instead of cockpit windows could they have just screens displaying what is happening using a camera? Is it for any useful purpose? Is it a structural burden or is it there for psychological purposes?

Why do cars have windshields? Why do glasses have lenses? You could replace both with video screens.

I don't think this is a duplicate. The suggested dupe asks is it possible, the answer to which is simply yes. This asks why is it not done, which has many reasons, some of which are provided in the suggested dupes answers but that doesn't make the question a dupe IMO.

@Notts90 A dupe is the same question, or, an existing question which yields the same answers.  The linked duplicate does address "why not".

@Simon there is information in those answers that gives some of the reasons that would answer this question but I think there are benefits to having more specific questions like this one to get better/more specific answers. Yes there is overlap but it's not exactly the same.

Screens are 2D. The human eye is an exceptional organizational designed to work best in the real 3D world. The information provided to the brain through the eye far exceeds that available from any display system; there will never be a replacement for that.

Yeah, it may be better. But what happens if the camera gets dirt (as everything else does in a flight) or if the camera fails for whatever reason? If the aircraft suffered an electrical malfunction and the camera was to turn off? Also, this is way more expensive than just fitting the window. The technology would also increase the weight of the aircraft. It's a good idea that I am sure that aircraft manufacturers have contemplated. It is just too impractical.

"this is way more expensive than just fitting the window": Can you explain why? I'd have thought a camera and a display are less expensive than the windshield anti-ice system alone.


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Is TGA analysis required for composites used in low speed subsonic aircraft?
I need to analyse the temperature drop effects on glass-fiber reinforced polymer (GFRP) used in aircraft subjected to icing, how can  it be done?
Does thermogravimetric analysis (TGA) help or is it only for high temperature material?


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How many axes are used in weight and balance calculations?
When doing weight and balance calculations on fixed wing aircraft is the transverse axis (wingtip to wingtip direction) used?  Or is only longitudinal axis considered?

Depends on the aircraft, sometimes all three are used, sometimes only one.

@Ron Beyer's comment is correct. Here's a little more information.
Generally speaking, in narrow-body aircraft, it's sufficient to calculate the CG only along the longitudinal axis. There may be limits along the lateral axis but which would only be exceeded with severely imbalanced wing fuel loads, and that is often handled by specifying a maximum fuel load imbalance.
Wide body aircraft typically require looking at the lateral CG as well as the longitudinal CG. However, the allowed lateral moment about the longitudinal axis may be high enough that the limitation comes into play only when carrying cargo. That's the case with 747-100/200/400 aircraft. As I remember, the max lateral imbalance for a 747-400 is 10,000,000 inch-lbs.
There are some fixed-wing aircraft that require looking at the vertical CG. The 767-300 does. I don't know about other 767 models. It's not an issue for 747s.
If you're interested in playing with CG movement on 747-400 and 767-300 aircraft, you can go to 747.terryliittschwager.com and look at worked examples or do your own.

Do you know if vertical CG was considered for external cargo 747's, specifically the shuttle transport?

@RonBeyer No, I do not. My guess (and it's only that) is that they certainly would have considered the vertical CG in the design for the shuttle transport; it was after all a heavily modified 747. I've seen, but do not have, Boeing manuals that detailed the external carriage of a spare engine. As I remember, though, there was no mention of a vertical CG check.


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Can a 737 yaw while parked due to wind?
How much wind force is required to push/rotate a 737 sideways while parked?

Welcome to aviation.SE! I don't really understand your question. Are you asking how strong the wind would have to be, to move a parked 737 sideways? Or how much the vertical stabilizer of a parked 737 flexes in the wind? Or something else?

Are you asking how much the aircraft could physically move, or how much force is applied to the aircraft by the wind? The wind loading is fairly straightforward, you can neglect the wings and just use the cross section of the fuselage and vertical tail. If you want to know if this would move the aircraft you need to know the coefficient of friction of the tires/pavement and the weight of the aircraft to figure out how much force would be necessary to overcome friction.

Hi thanks, yes a bit of a strange one but basically take a stationary 737 and a  full crosswind of say 10 knots... the vertical stab being 26.44 m2 in area and the force the wind would have on it in a turning effect, how could you work out how far left or right the aircraft tail or nose could be turned or moved by the wind, thanks

Unless the nose wheel is on ice or a very low friction surface, this isn't an easy answer. If you run through the calculations though, you'll find it takes a very significant cross wind to move the aircraft nose (skid on pavement), probably in the order of 75+ knots. It also depends on loading, if the aircraft is loaded nose-light, it will be easier, but if its loaded nose heavy, it will be harder.

The answer will depend on wheel loading and the coefficient of friction for the supporting surface.

@ymb1 - Everyone else is running away, and the guy in blue is trying to push it back. Major props for the effort, man in blue!

Slightly related (with a B747): "The next day the plane was found to have also rotated about 45 degrees from its original position." https://youtu.be/cHhZwvdRR5c

From own experience some years ago an empty B737-300 with ~2500kg fuel in the tanks can be moved by crosswinds as low as 55kt. Surface was a regular dry concrete apron with no slope. That's why my companys GOM (ground operation manual) required ballasting the aircraft with max fuel in high winds and chocks on the main wheels both sides.


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Weather Reporting of wind direction
General weather reporting for wind direction in Australian Standard the degrees in wind direction identifies where the wind is coming from.
Is it the same around the world?

Yes, it is generally used that the broadcast wind is the magnetic heading for the direction the wind is coming from. This is because standard runway numbering is magnetic direction and pilots can easily determine the most appropriate runway given the wind.

I always thought it was because phrases such as "north wind" and "west wind" have always referred to winds from the north and west, respectively, and the most obvious convention is to report a "west wind" as 270.


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How to demonstrate that an elliptical circulation distribution can be induced by an elliptical chord distribution?
I intend to demonstrate that a wing with an elliptical chord distribution, without torsion ($\alpha(z)=\alpha=\text{const}$) and constant section ($\beta(z)=\beta=\text{const}$, and $a_{2\text{D}}(z)=a_{2\text{D}}=\text{const}$), can induce an elliptical circulation distribution.
Well, for an elliptical chord distribution, the chord is defined by:
$$c(z)=c_C\sqrt{1-\left(\frac{z}{s}\right)^2}$$
where $c_C$ is a constant.
On the other hand, the circulation distribution is defined by:
$$\Gamma(z)=\frac{1}{2}a_{2\text{D}}V_\infty c(z)\left[\alpha+\beta+\alpha_i(z)\right]$$
So, by substituting $c(z)$ on $\Gamma(z)$, we get:
$$\Gamma(z)=\frac{1}{2}a_{2\text{D}}V_\infty \left[c_C\sqrt{1-\left(\frac{z}{s}\right)^2}\right]\left[\alpha+\beta+\alpha_i(z)\right]$$
which can be written as:
$$\Gamma(z)=\Gamma_C(z)\sqrt{1-\left(\frac{z}{s}\right)^2}$$
where $\Gamma_C(z)=\frac{1}{2}a_{2\text{D}}V_\infty c_C\left[\alpha+\beta+\alpha_i(z)\right]$
So, to demonstrate that an elliptical chord distribution can induce an elliptical circulation distribution, we would also need to demonstrate  that $\alpha_i(z)=\alpha_i=\text{const}$, to make $\Gamma_C$ a constant.
I don't know how to do this last step. How would you do it?
Note that:
$s:=$ semispan of the wing,
$z:=$ transversal coordinate of the wing, it goes from $-s$ to $s$,
$\alpha:=$ geometric angle of attack,
$\alpha_i:=$ induced angle of attack,
$\beta:=$ symmetric of the null lift (geometric) angle of attack,
$a_{2\text{D}}:=$ airfoil lift coefficient slope,
$V_\infty:=$ freestream airspeed.

First, let's transform the coordinates to polar coordinates for simplicity by introducing a variable $\theta$ such that:
$$z=-s\cos{\theta}$$
In this coordinate, the chord distribution is then:
$$c(\theta)=c_C\sin{\theta}$$
Let's write the unknown lift distribution of the bound vortex as a Fourier sine series, again in the $\theta$ coordinate:
$$\Gamma(\theta) = \sum_{n=1}^{\infty}A_n\sin{n\theta}$$
Through Biot-Savart, the induced AOA can be computed from the trailing vortices, such that:
$$\alpha_i(z) = \frac{1}{4\pi V_\infty}\int_{-s}^{s}{\frac{(d\Gamma/d\zeta)}{z-\zeta}d\zeta} = \frac{1}{4\pi V_\infty}\int_{0}^{\pi}{\frac{\sum_{n=1}^{\infty}{nA_n\cos{n\theta_0}}}{\cos{\theta_0}-\cos{\theta}}d\theta_0} = \frac{1}{4V_\infty}\sum_{n=1}^{\infty}{nA_n\frac{\sin{n\theta}}{\sin{\theta}}}$$
Now collecting everything together and put into your original equation, we have:
$$\Gamma(\theta) = \frac{1}{2}a_{2D}V_{\infty}c_C\sin{\theta} \left[ \alpha - \beta - \frac{1}{4V_\infty}\sum_{n=1}^{\infty}{nA_n\frac{\sin{n\theta}}{\sin{\theta}}} \right] = \sum_{n=1}^{\infty}A_n\sin{n\theta}$$
The key, at this point, is to realize that each $\frac{\sin{n\theta}}{\sin{\theta}}$ is linearly independent to the others, so in order for the other terms to satisfy the above relation for all $\theta$, they must be simultaneously zero. That is,
$$A_n = \begin{cases}
      A_1 & n=1\\
      0 & n>1\\
    \end{cases}$$
So now the circulation and chord distributions have the same form, which are elliptical.


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Where is the mistake in this air density calculation?
I intend to compute the air density, $\rho$ at an altitude of $h=15000ft=4572m$, with an ISA deviation of $\Delta_{ISA}=18^{\circ} F=10\,K$.
Well I first computed the real temperature of the air at $h$ altitude with the following ISA temperature realtions:
$$T_{ISA}[K]=288.15-0.0065\,h[m]=258.432\,K$$
$$T=T_{ISA}+\Delta_{ISA}=268.432\,K$$
I then computed the temperature altitude, i.e. the $ISA$ altitude for a temperature $T$:
$$h_T[m]=\frac{T[K]-288.15}{-0.0065}=3034m$$
With $h_T$ computed I got the air density by the respective ISA relation:
$$\rho[kg/m^3]=1.225\left(1-0.0065\frac{h_T [m]}{288.15}\right)^{4.25588}= 0.906kg/m^3$$
The problem is that this value is different from what I got here ($\rho=0.742 kg/m^3$)
What did I do wrong?

The mistake is subtracting the ISA temperature at sea level from the ISA+18°F temperature at altitude in step two. If your atmosphere is heated up, it needs to be hotter at sea level as well, or your lapse rate in the first step and the denominator is wrong.
First calculate density for ISA conditions. This gives 0.7708 kg/m³.
Then used the ideal gas law to see how a 10 K temperature increase will reduce density at constant pressure. The density ratio is inversely proportional to the temperature ratio, so the result is 0.7421 kg/m³.
When checking your result, input only 10 K into the calculator, not 18 K !!!

@ÉlioPereira: Only by assuming a 18 K temperature difference!

But pressure changes with temperature. Why did you consider that a deviation in temperature only affects the density of air?

I didn't know that. So the 15000ft is a pressure altitude and not an height. Thank you.


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Can IATA code for an airport be more or less than 3 letters?
I got an exercise where IATA codes are involved - exercise's specification says those codes can be 2-4 letters. After a few minutes of research I see that those codes should be 3 letters, so can IATA code for an airport be more or less than 3 letters?

IATA codes for what? Different kinds of codes have different number of letters.

question edited

IATA codes for airports have 3 letters, while IATA codes for airlines have two.

IATA codes for meals have 4 letters. I don't know of any airport though that does not have a 3 letter code.

IATA themselves say it's a three-letter code:

Airlines and CRSs [computer reservations systems] may request the
  assignment of a unique three-letter code to identify an airport

The UN trade locations coding also says the same thing:

when a three-letter code is used alone to indicate a location, it
  designates the name of an airport or location as adopted by IATA
  (whose code only has three letters)

According to Wikipedia, IATA has several different types of code that can be from one to four characters, but airport codes specifically are always three based on all the sources I mentioned.


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Are carbonated drinks considered dangerous goods for air freight?
I understand carbonated drinks in allowed in cabin, but, is it allowed in air cargo transportation, when you ship in bulk?

Why would it be dangerous?

@egid Are soft-drink bottles strong enough to cope if the plane depressurized at FL350? I guess so but, if not, you could get a lot of CO2 released in an already dangerous situation.

@DavidRicherby Chemistry SE suggests that they're already nearly 3.5 atmospheres, so I doubt 0.7 additional atmospheres will make that much difference.

Funnily, the few times I tried to pack carbonated cans into my checked in bags they seem to have exploded / imploded and created a royal mess. Never figured out why!

Each operator is of course entitled to apply stricter regulations, but there's no restriction against carbonated drinks in the IATA-DGR (based on the ICAO TI, which sets out the framework for dangerous goods regulations).
The maximum amount of pure CO2 an operator may transport in approved packaging (iaw. IATA-DGR) is:

In gaseous form (UN1013): 150 kg on cargo only aircraft, and 75 kg in passenger/cargo aircraft.
In liquid form (UN2187): 500 kg on cargo only aircraft, and 50 kg in passenger/cargo aircraft.
In solid form "dry ice" (UN1845): 200 kg for both cargo and passenger/cargo aircraft

of course dissolved CO2 in carbonated drinks falls under neither of the listed categories...

@jwenting The actual CO2 in a carbonated beverage is in gaseous form. However, a lot of the CO2 that gets dissolved into a carbonated beverage ceases to be CO2 at all, but, rather, H2CO3. It seems reasonable enough that this shouldn't even be counted under the CO2 transportation limits, since it isn't actually CO2.

@reirab That's like arguing that you should be allowed to take a block of ice through security because it's not a liquid. Sure, the carbon dioxide is in the form of H2CO3 while it's in the bottle, under pressure but, once that pressure is released, it splits back into water and CO2.

@DavidRicherby Since the regulation itself specifically treats it differently depending on state in this case, it seems reasonable to assume that a state not listed wouldn't be covered, especially when it's not even the same chemical at that point. I'd guess that there are probably different regulations concerning carbonic acid, in which case those should be followed.

Thanks, good to know. Are there any easiest way to find out if an item fall under the  IATA-DGR, besides I ask them again in Aviation SE?

@Ryan The manual is available to the public in different formats on iata.org, it is however a bit pricey just to satisfy curiosity :)

@reirab Actually, the whole carbonic acid thing is a red herring. The term "carbonic acid" is used to denote both the substance H2CO3 and a solution of carbon dioxide in water (CO2+H2O). When carbon dioxide is dissolved in water, the great majority of it is literally that: carbon dioxide dissolved in water. Only a tiny fraction reacts to form H2CO3.

This Chemistry.SE question says a can of soda contains about 2.2 g of CO2.  If we take the 75 kg limit, that's about 34000 cans of soda.  This much soda will weight about 12 tonnes.

hush, @NateEldredge and your silly little facts, we're having a good argument here!  :D

Suppose someone had a lot of soda they really needed to move in a hurry for some strange reason. 12 tonnes of soda could certainly be shipped as air freight if you had money to burn.

@DavidRicherby: On the other hand, the portion that does react is very important (for instance, it's either the whole reason, or a major part of the reason, depending on the specific drink, that carbonated beverages are acidic).

I was looking this up, because I am taking IATA DGR training.
Per IATA carbonated beverages are not subject to dgr regulations as they are foodstuff
IATA ref 3.2.2.4.2 (very slow pdf download)

3.2.2.4 Exemptions
3.2.2.4.1 Gases of Division 2.2, are not subject to these Regulations if they are transported at a pressure less than 200 kPa at 20°C and are not liquefied or refrigerated liquefied gases.
3.2.2.4.2 Gases of Division 2.2 are not subject to these Regulations when contained in the following:
(a) foodstuffs, including carbonated beverages (except UN 1950);
(b) balls intended for use in sports;
(c) tyres which meet the provisions of Special Provision A59; or
(d) light bulbs, provided they are packaged so that the projectile effects of any rupture of the bulb will be contained within the package.

No, Carbonated drinks are generally not dangerous, and are labelled in such a way to let anyone who is transporting them know the contents of the carbonated drink. Before they become dangerous though, other factors would have had to happen that would be considered even more dangerous; The airline would have had to have a sudden change of pressure, such a change would cause a loss of consciousness for all those travelling on board the airline, and doing so would release CO2 from the cans.

and doing so would release CO2 from the cans. [Citation needed]


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Main battery is discharging with alternator running

In my Cessna 172S, we had a problem about 'main battery discharge' after an hour of flight.
We tried doing wiring test on the connection at the master switch which ran from the alternator and performed a continuity test following the wiring diagram, but the result of the continuity test is ok and the problem still exists.
Also, we changed out a new ACU and alternator, but that didn't resolve the problem either.

Is the battery just going bad?

Is this your personal aircraft, or is it a club plane? If it's a club plane, or a rental, report it and let the club's mechanic deal with it.


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Live streaming cockpit to air traffic controllers
I was reading about the missing of the Malaysian flight 370 and was wondering if it would have been helpful if the cockpit would have a live streaming service where they live stream what's happening to the tower. I think it's too much bandwidth and data but was wondering why people haven't implemented.

How would that help? The pilot would have turned that data link off too... Can you imagine the workload increase if ATC had to not only monitor communications, but watch video streams of cockpits?

Sure. A stream of security-cam footage might have been helpful.
It wouldn't have been helpful for the Tower, certainly. The Tower is mainly responsible for ensuring safe operations on the runways of an airport. Any airspace they do own is unlikely to extend beyond five miles in any direction of the airport.
And it wouldn't have been that helpful for the radar controllers either; the most it would have done is confirmed whether the pilots were unresponsive on purpose or due to some mishap. It wouldn't have changed the fact that they were unresponsive on the radios or that they changed their flight path.
It wouldn't have been helpful for Malaysia Airlines dispatchers or operations personnel either, except to (again) point to a possible reason for what was occurring, and for the same reason it wouldn't have been helpful for security personnel.
It might have been helpful for the lawyers, to assist in assigning blame for the incident; this could have had an effect on how much monetary compensation the next-of-kin received and who was responsible for paying it.

In general, such a livestream would be useful "in the moment" only in very limited situations. There are only two I can quickly think of:

The aircraft is in trouble and the pilots are talking to ATC, but ATC does not yet understand the magnitude of the trouble or the magnitude of the pilots' workload. Viewing the livestream might help them understand better what the pilots are facing and avoid asking unnecessary questions.
In a talk-down landing, having a video feed to confirm that the proper controls are properly selected could be very valuable. However, (1) it is unlikely that a general security-cam view of the flight deck would provide the resolution and focus necessary to confirm that and (2) while there have been examples of talk-down landings they are exceedingly rare, and even more so on airline-transport category aircraft.

Having such a system available for live viewing means there needs to be someone trained in what "normal" and "abnormal" flight deck activity looks like, and they need to be actively monitoring video feeds from (ideally) more than one aircraft at once (or else things will get even more expensive than such a system would already be). This person should not be a controller, who already has a lot of other duties, so now you have to hire a wholly new position or else put the responsibility on what is, in all likelihood, an already-overworked dispatcher at the airline. And as you say, such a system would cost a lot of money in equipment and bandwidth, and coverage over the open ocean would be limited to (even more expensive) satellite links.
Besides, every electrical system on an aircraft needs to be wired into a circuit breaker, for very valid safety reasons. Fires on an aircraft are much more common than hijackings, and generally more dangerous. And as Ron Beyer points out, if the pilots are able to switch off a feature (which there are valid reasons for doing; see also this answer regarding the transponder for the flight in question) then having that feature is not a safeguard against a malicious pilot.
So in all, such a system would provide extremely limited benefit in exchange for quite a lot of added complexity and cost. That is why it does not exist. (That and the fact that pilots' unions oppose the idea as an invasion of privacy for little added benefit.)


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Are modern commercial airliners flyable without operational computer systems?
Do modern commercial airliners have manual control capability to fly and land if electric power remains on but all computer systems and digital communication/guidance fails?

Your question is really generic. You can restrict it to some aircrafts types as some modern airliners (e.g. the A380) rely on electronic systems only whereas other (e.g. A320 with rudder and trim) not. (I really have no idea for Embraer or ATR airliners)

For a non-fly-by-wire airliner, yes.  It's just another airplane in the end and as long as there is hydraulic power you can fly it.  If the weather is clear you don't need any electronic aids at all, if you know how to fly in the first place.
For FBW, where computers control the controls and the pilot is just telling the computers what he wants to do, they are dependent on multiple levels of electronic redundancy to account for those sorts of breakdowns.

John K answer is a great summary, just to complement you should consider the probabilistic safety of systems.
In a conventional airliner you depend on the hydraulic system, on cables, on mechanical transmission etc
On FBW (flight by wire) you depend on electrical power, electrical and/or hydraulic servo controls, on multiple AND/OR data communication buses, on multiple AND/OR computers...
On FBW you have a high level of redundancy which allows for multiple way of implementations such as:

On the same surface ( aileron, elevator etc), you have several servo controls some of them are autonomous requiring only the electrical generation and the command from the computers, and some other dependable of the central hydraulic system and of the computer command
several computers may command every servo control
multiple parallel data communication such as ARINC 429 and ARINC 629 and fiber optic etc
In addition to the conventional multiple layer of electrical generation you have Multiple emergency power such as central batteries, and local batteries for the FBW

Considering the safety of modern airliners for instance the B777 which is nearly a full FBW aircraft, it had no accident.
What makes the superiority of FBW airliners is the redundancy of equipment and the redundancy of implementation.
The weakness of conventional airliners is the lack of redundancy, considering for example a dépressurisation that deforms the deck and prevents the mechanical cable transmission to the elevators servo control, we get into a hopeless situation, while with FBW you will just do a standard emergency descent.
In the question, the assumption of loosing all communication between computers while electrical power remains is probabilistically impossible.


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Do turbine and compressor rotate in opposite direction?
I have heard that they do rotate in opposite directions to maintain the  equilibrium of the engine. That is the net force generated by the rotation is nearly equal to zero. But, I don't think the force generated by both compressor and turbine is exactly equal and gets canceled. Am I right?
$$
F_{net}=  F_{c} - F_{t} \approx 0
$$
If I am correct up to this point, how do the compressor and turbine shafts connect so that they move in the opposite direction? What additional force is produced to maintain equilibrium? By combustor? A picture would be much appreciated.

Not usually, they are almost always fixed to the same shaft so that the turbine drives the compressor directly. That means, in the same direction.
The high-bypass geared turbofan does exist, but is unusual. And even there, the fan provides minimal compression; one or more further compressor stages are still mounted on the main shaft.
You may perhaps be thinking of the two-spool turbofan, in which a first-stage compressor, inner shaft and second-stage turbine are followed by a second-stage compressor, outer (cylindrical) shaft and first-stage turbine. The two spools spin at different speed.
But even here, they usually turn the same way. A few designs do buck the trend and spin them opposite ways, see Why are contra-rotating jet engines so rare?
The turbine blades tend to resist the air flow and deflect it tangentially, while the compressor blades tend to push it backwards and (if they spin the same way) deflect it in the opposite tangential direction. The fore-aft effects in part cancel out, leaving the rotor in a state of net tension (and net thrust on the fan/compressor blades). In steady operation at constant revs, the torque effects are (of necessity) equal and opposite. They do create a net torque on the shaft, which it must be strong enough to resist. Stator blades may protrude in from the casing to counter the tangential gas flows; again, if properly designed these will result in zero tangential flow and the casing will also experience some twisting but, overall, the air will not.
A counter-rotating fan pushes the air tangentially in the same direction as the turbine. If this is not carefully countered by compressor blades attached to the turbine spool, it may create wasteful spinning and turbulence in the engine wake. The fan transfers its torque through the gearbox to the driving turbine, and vice versa. Because the gearbox reverses the torque, perhaps paradoxically the torque acts in the same sense on both shafts. But overall, whatever the configuration, torque cannot be transmitted between the spool and the casing unless the engine revs are changing. The main difference is that the thrust of the fan is decoupled from the drag of the turbine. Depending on where the thrust bearings are located, this may affect whether the thrust loads are carried by the shaft, and what their path through the casing is to the wing mounting point.

@Bianfable Thanks for pointing that out, I have amended my answer.

If they don't then how are the force resolved in the gas-turbine engine? Won't there be high stress in only one direction on the pylon or the whole engine?

Can you explain what forces and stresses you are asking about?  (longitudinal, rotational, etc.)

@Auberron OK, I have added some general remarks about that. OK yet?


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What is the effect of a taildragger on P-factor?
By looking an image of P-40 Warhawk with taildragger (and other vintage warplanes), one can conspicuously find the propeller disc of this aircraft isn't perpendicular to the relative airflow which induces P-factor.
Doesn't this design forces the aircraft to further left which causes difficulty for pilot to direct rudder to right?

Is this the reason why all the modern airliner's (including single engine aircraft like Cirrus SR22, Kestrel K-350) fuselage isn't made inclined?

Does this answer your question? Why tail wheel rather than tricycle?

No, this question also addresses p-factor.

I do not understand, you voted saying that the other question is a duplicate of yours, but commented otherwise. Also, I do not understand your reference to the p-factor, since it is a function of the AoA at the prop, not the mounting angle.

@ymb1 ah, I see.

@ymb1 -- title needs some help.  There seems to be a verb missing.  Perhaps what was meant was "What is the effect of being a taildragger rather than having a tricycle gear configuration, on the effect of P-factor?"  Or, "What is the effect of P-factor on a taildragger?"  Or-- ?? -- something like that?

By inclined, do you mean that the conventional (taildragger) landing gear configuration angles the propeller? If that is in fact your question, know that the propeller is not “inclined” in flight any more than a tricycle landing gear aircraft. Straight and level is basically (very roughly) the same between the P-40 and the SR-22. And, P-factor is only induced when the aircraft has sufficient forward motion. At very low airspeeds, there is hardly any P-factor.
In a conventional (tail-dragger) landing gear aircraft, once you are above the customary taxiing speed (the sufficient airspeed mentioned earlier), you pitch the nose forward. This raises the tail. In effect, you eliminate the “incline” in question. You are, at that point, almost flying the aircraft as if it had tricycle landing gear.
The conventional landing gear was the norm in those days. And, they were more conducive to landings on unimproved runway surfaces. Although, the conventional gear is more advantageous on non-hardened surfaces. Its biggest disadvantage is its difficulty in steering at low speeds. The lack of a steerable nose wheel means you must rely on differential braking only at low speeds and the rudder at medium to high taxi speeds.
This leaves you prone to what is called ground loops. A ground-loop is caused by the center of gravity being positioned behind the point of lateral rotation (the main landing gear in this case). Any over use of differential braking or miscalculation of the leverage applied by the long leverage arm of the rudder can cause the aircraft to violently and uncontrollably spin around. Think of trying to steer your car while driving at high speeds in reverse.
The old adage is that you have to fly a taildragger from the moment you leave the hangar till the time you park it to put it away. Tail-draggers require you to be a lot more adept on the rudder.
It also leaves you vulnerable to prop strikes due to nothing stopping the nose from over-pitching. Something very easy to do by applying the brakes too hard during roll out from landing.

Thanks @Dean F for your lovely answer. Here is a video  that shows propeller isn't inclined at the time of take-off.

Just a couple notes here:  A taildragger pilot will often take off from a "three point" attitude, in which case, P-factor continues more or less continuously from the time power is applied to the time the aircraft lowers it's angle of attack.  If the pilot raises the tail for takeoff, P-factor will disappear while the AOA is lowered, but the pilot can also expect precession to pull the nose to the left while the nose is brought down, in proportion to the pitch rate, requiring more right rudder in the transition.  Harvey Plourde goes through this in detail in "The Compleat Taildragger Pilot"


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What causes air to move to combustor?
I know the pressure is highest at end of compressor. But what forces air to move low to higher pressure? Am i missing out something? I have heard seals are used to prevent leakage of air from compressor to forward section of engine.

Your question is confusing. You acknowledge that pressure is highest at the final stage of the compressor, yet seem to think that the airflow could somehow reverse direction, or flow from low to high. If the pressure is lower in the combustion section then that's where air will flow...

And any seal preventing forward flow would also restrict aft flow, right?

What forces air to move from lower to higher pressure?

In the compressor, the pressure continuously increases from first stage to last stage, that's the purpose of the whole thing. But your question is valid: Why would air move from the first stage to the second stage where the pressure is higher?
The reason is indeed the rotating blades of a stage push air to the next stage. Energy extracted by the turbines is used to rotate the compressor blades and create a relative wind. Air is accelerated over the blade and like for a wing, a downwash force is created. This accelerated air is then slowed down by the next stator vanes to convert the additional velocity into additional pressure, and maintain velocity nearly constant in the compressor (else efficiency would decrease), as visible in tsg's answer.
Stop the blades and air goes back until pressure equalizes in all stages.

I have heard seals are used to prevent leakage of air from compressor
to forward section of engine

Correct, there are seals. But while the air moves towards the combustor within the compressor annular duct, under the effect of blades rotation, seals are here to prevent it from getting back to previous stages by escaping the annular duct, and leaking below the stator vanes (between the rotor drums and the inner shrouds of the vanes).
The fixed vanes plane is actually the plane where pressure increase occurs. So air will try to return on the front side of the vanes, towards the rotor blades using the interstices (orange flow), but it will be blocked by seals.
The seals are highlighted in yellow on the picture below, they rub on the inner shroud of the stator vanes (actually for the engine shown, a honeycomb ring fixed inside the vanes shroud provides an abradable track to the labyrinth seal:

HP compressor interstage labyrinth seals (CFM56-7B), source
These seals are required because a small gap exists between the rotor and the stator platforms forming the inner wall of the duct.

The compressor, of course.
The pressure inside the combustor is lower than at the end of the compressor. How is that achieved? By making the combustor wider than the compressor's final stages. That's why, on a cross-section, you see the gas path flare-up from the compressor to the combustor.
The combustor does not increase the pressure. After the compressor, the pressure only ever drops; this loss of pressure is what propels the air from the front of the engine to the nozzle.
So the pressure decreases throughout the combustor. What the flames do is increase the gases' energy, which is stored as temperature and velocity. Once the hotter gases reach the turbines, then, their pressure is "worth more" than for cold air. This way, the torque they put into the turbines as they expend their pressure is greater than the torque the compressor required to compress cold air.
Finally, some of the energy remains as velocity to help propel the aircraft.

Diagram shamelessly stolen from here. It was also used in this question, which is useful for additional detail.

Just remember what pressure is - force divided by area - so a difference in pressure forces the air (or other fluids) to flow in its gradient.
You do need seals to prevent flow in an unwanted gradient - but they mostly just "choke" the flow, not completely prevent it.

What prevents air to leak completely?

@Auberron that's a separate question

@Auberron, air doesn't "leak" forward or backwards, it flows through the engine continuously.

@Auberron speed of sound prevents air leaking completely


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Why and when to use flaps?
On a commercial airliner, what is the purpose of the flaps? I understand that depending on which direction they move in, they can be used to decrease airspeed, but I don't know when flaps are used. Are flaps used before takeoff, after takeoff and on approach?

Have you reviewed the other questions tagged [tag:flaps]?

@GregHewgill I was out on my phone when I wrote this question. I did look for the flaps tag but I must have missed it. I will look again.

Hmm, when you say "which direction they move in," do you mean the 'flaps' you see after an airliner touches down on the runway that stick up from the wings? I believe those are not flaps, but rather spoilers, or air brakes.

Also, spoilers DO directly affect (are designed to) airspeed and are used to slow the plane's speed down.

@CGCampbell Not true at all. Spoilers affect rate of descent, and attitude affects airspeed.

Ooof, must always research. Spoilers reduce lift.

Flaps change the camber of the wing's airfoil. This in turn changes the zero-lift angle of attack, increases the maximum lift potential and, in case of fowler-type flaps, the wing's area.
Airliners use flaps on both take-off and landing, and most of the time they are only partly deflected. Full deflection is only set on final approach to reduce the landing speed as much as possible.
When simple camber flaps are used, the drag increase is negligible for small deflections (±10° for a 20% flap), and they can be used for both take-off and landing. Other types of flaps are designed to increase drag (split flap, Zap flap) and should be used only for landing. If flight speed stays constant, a flap deflection will only change induced drag if it redistributes lift over the wingspan. Modern gliders use camber flaps inboard and flaperons (a combination of aileron and flap) outboard, and changing flap settings will not affect induced drag.
Generally, the purpose of flaps is to shift the minimum airfoil drag to the desired lift coefficient $c_L$. Especially laminar airfoils have a pronounced range of lift coefficients with low drag. The pilot should attempt to stay within this region of minimum drag, and flaps give him/her the means to achieve this. See below for an example of an airfoil at a Reynolds number of 1 Mio.

Flaps for lift increase work the same way, but change camber much more. Normally, they need to be combined with leading edge devices (slats) which change camber at the airfoil's nose to unfold their full potential. Below are some examples for flaps, in combination with proper leading edge devices. Note that the solid outline of the fowler flap is the position for landing, while the dashed outline is the position for take-off. $\delta$ is the relative chord of the flap.

Junkers flaps were the best way to control big aircraft in the days before more elaborate force compensation mechanisms were developed, and split- and Zap flaps were very popular in the 1930s. They are still used on smaller aircraft today. The simple Fowler flap is used on the C-130, and airliners use slotted flaps like the ones in the lowest two rows of the table above.
Besides increasing drag, powerful flaps change the pitching moment of the wing by shifting the center of lift backwards, which requires sufficient tail size and control power. On airliners, the incidence of the full horizontal tail can be adjusted in order to create enough downforce to counteract the pitching moment. Note on the picture of the An-70 below that the stabilizer has a slat which operates in the opposite direction to that of the wing.

I wish that I could give you 10 up-votes.  Great answer!  :)

@Lnafziger: Thanks for the kind words! I hesitated first if I should post a long answer to an old question, but the line "Flaps increase both lift and induced drag" in the only answer so far bugged me.

@Peter Kämpf, can we say that flaps are also deflecting the airflow downwards and increasing lift this way? In other words they introduce some sort of Newton's 3rd law principle?

@ParadigmPilot: Not only the flaps, the whole wing is doing this constantly while in motion. The flaps only increase this effect.

Flaps increase lift and drag by changing the camber of the airfoil, and allow lower airspeeds. There is also an effect of shifting the center of pressure (C of P) aft. There may be an immediate decrease in airspeed due to the increase in drag (see the Wikipedia article linked below) until the aircraft regains stability by nosing down. There are some incorrect discussions online, such as one answer on this page.
Flaps are frequently used when landing because they lower the stall speed and increase the angle of descent. The pilot will also have better visibility due to the lower nose position caused by the aft C of P.
Flaps increase both lift and induced drag, but a small amount (e.g. 10 degrees) of take-off flaps may provide enough extra lift to allow the plane to become airborne sooner, without a large increase in drag, allowing for a shorter takeoff roll. Whether takeoff flaps can be used is declared in each aircraft's standard operating procedures.
I found these resources that I like:

How to explain flaps to a Student Pilot?
Flap (aeronautics) on Wikipedia

The wikipedia article pretty explicitly states that flaps do in fact affect airspeed...  When they are deployed they do increase the drag of the wing, which does slow the craft down.  I doubt it's as effective as an airbrake, but still...  Everything else you said appears correct though.

@JayCarr it is a delicate subject, if you increase drag, you need more thrust to maintain airspeed and altitude. Flaps are usually extended during descent, or while standing still on the ground and not in level flight, so the effect on airspeed is not immediately apparent.

Updated, thanks Jay Carr

@tar; some aircraft will return to trimmed speed all by them self, no manual pitch down required; and I imagine autothrust will counter to the drag with thrust, so it's not that simple :)

@Articuno; I suppose that depends on your definition of being equal. Autothrust still enabled? Trimming? The DA20 Katana will happily maintain airspeed when you extend the flaps. (The immediate effect is airspeed decrease, sure, until it settles in with an increased descent rate and the airspeed returns to the trimmed speed, thus extending flaps all other things being equal increases the descent rate and affects pitch attitude) Of course the DA20 is no airliner, but the point remains, it's not that easy unless you define what should remain equal.

@falstro - "All other things" are to remain equal in the example.  The overall effect of flaps on the system is to increase drag and lift.  Yes you can, and probably should, compensate.  But the overall effect of the flaps, and the flaps alone, is drag + lift.

@JayCarr; exactly, they do not necessarily decrease airspeed.

@tar - No problem.  Flipped my vote from down to up, good answer :).

Lower pitch attitude is not due to aft CofP, but due to increase in lift. Because in stable flight the airplane needs lift exactly to oppose weight, the increase in lift at given AoA is compensated by flying at lower AoA and provided the glide slope remains the same lower pitch.

Induced drag from deploying flaps is inversely proportional to airspeed (see the Di curve here: http://qph.is.quoracdn.net/main-qimg-3a303d1f0dcee8a6ffef124a9f8aecb6).  At low (landing) airspeeds, the amount of drag from flaps in the landing configuration is near its maximum.
If you trim the aircraft for hands-off flight, then deploy landing flaps without touching the yoke or the throttle, the pitch of aircraft will change to keep the aircraft flying at same speed.

@rbp It's true that the induced drag drops with increased airspeed (it's actually inversely proportional to the square of the airspeed,) but it's not true that total drag is at a maximum at low speeds. The opposite is usually the case, as parasitic drag eventually dominates as airspeed increases and it is directly proportional to the square of the airspeed. In the graph you linked, note that both Dp (parasitic drag) and Dt (total drag) increase with airspeed after the point where Di equals Dp.

OK, well I prefer to land at or near 1.3Vso, not at Vne

The purpose of trailing edge flaps and leading edge slats is to alter the wing profile in such a way that it generates more lift at low airspeeds, while lowering the stalling speed. This makes it possible for the aircraft to take off and land at lower speeds than would otherwise be necessary. From the design perspective the inclusion of flaps makes it possible to optimise the basic wing section for the best performance in cruising flight.


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Why do we no longer have a navigation officer on modern commercial flights?
In the olden days there used to be a navigation officer in commercial airlines who had the tasks of navigation and radio communication. But, in modern commercial airliners there is no navigation officer.
Do the pilot(s) take the additional responsibility of radio communication and navigation? Or is the navigation part now done by onboard computers and systems?
If the electronic navigation system fails, are there any backup plans? Are physical maps and a compass then used to determine the direction and position?

There are a considerable number of answers to this question here, which I found with a Google search for "airplane navigator": http://boards.straightdope.com/sdmb/archive/index.php/t-503882.html

Navigation has gotten much simpler over the years.  Initially, navigation would be done by a combination of dead reckoning, looking out the window for landmarks, and for night or long-distance flights, by celestial navigation (many old airplanes have a window at the navigator's station specifically to let the navigator see the stars).  This took quite a bit of time and effort, so the pilot couldn't do it and also fly the airplane.
In the 1920s, radio navigation was added to the navigator's toolkit, but finding bearings to beacons and calculating a location was still an entirely manual process.  Additionally, radio beacons didn't cover the entire world, so older navigation techniques were still needed for most flights.
In the 1950s, fully-automatic radio direction finding systems became common and navigation beacons were available across most of Europe and North America.  At this point, the workload involved in navigation dropped to the point that the pilot could also navigate: plot out the sequence of NDBs to follow ahead of time, then tune the radio and follow the navigation display.
Very recently, navigation has been integrated with the autopilot to the point that the pilot can punch in a sequence of waypoints and have the airplane fly the route entirely on its own.
And yes, even today if the electronic navigation system fails, the fallback is map, compass, and slide rule.

But the sextant has been removed from the fallback, I guess.

I would note also that "the electronic navigation systems" consist of a bunch of different independent systems (NDBs, VOR/DMEs, GPS, INS) and the chance of all of them failing is negligible barring, say, an EMP. Of course, in the EMP case, your first concern will probably be that your engines just stopped working, thanks to very unhappy FADECs. To further complicate matters, the pax will be complaining of dead IFE systems and PEDs. :)

...and no lights, no AC, sudden quietness in the cabin as the engines have stopped....

@BurhanKhalid ...followed by screaming.

The other answer to this is just how I would have said it however I will respond to your second point of electronic failure and what you can do.
First off I am sure you know that airplanes carry a great amount of redundancy to avoid this and the changes of everything failing is almost 0 but lets say it does happen. I will go through what you can do and what backups you have. First lets talk GA VFR situations.
I am only VFR rated and fly only single engine planes. When it comes to navigation I have 2 options, paper maps, and no the electronic flight bag which I have just moved to. When I fly i carry my iPad (with external GPS receiver) and my iPhone. Both have the east half of the country worth of maps and procedures/info downloaded to them and independent GPS receivers. My pre-flight starts the night before making sure I have everything charged up. Now I'm in the plane my iPad overheats or crashes, I move to my phone, buys me another 30 min then it crashes, I move to my paper maps, compass and stop watch. As for stop watches I usually have 3, a chronograph on my watch, a hand held mechanical, and the one mounted in the plane (some times there are 2 mounted depends on what I am renting that day). This allows me to fly dead reckoning to my destination using my map, plotter and flight computer. Again doing some guesstimating in my head and some quick calculations. Now lets say for some reason none of my stop watches are working, my maps flew out the window, and I dont know any of the VOR frequencies in the area.
Look out the window until you see a landing strip, then buy some new maps...
Now lets move to some bigger planes. Large planes carry numerous electronic navigation aids. Along with using GPS, they have inertial navigation systems which are capable of working on their own. Although inaccurate (by today's standards) they will tell you how far/fast you have traveled from which you can extrapolate location. Since commercial planes fly under IFR most of the time, assuming one of the radios on the plane is functional ATC can guide them down assuming all of their equipment is no longer working. Again in the end of the day physical maps will get you where you need to be although I dont know if large planes carry VFR maps along with their IFR route charts.
Again the key here is redundancy and the fact that its highly unlikely that all systems on a plane will fail simultaneously. That being said even if it does happen the pilot should have a fair idea of where they are/where they are going and should know a general compass heading to divert to another airport.
Historically navigation equipment was also more cumbersome than the iPad I have on my knee board and knowledge aside, a navigator simply provided another set of hands and eyes to operate the devices which were often mounted behind the pilot. There is only so much display area on an aircraft dash for gauges and devices.
For what its worth its also worth noting that flight engineers are on their way out now. The 747 which once required an engineer eliminated the need with the 747-400 variant by including glass cockpits making everything available to the pilots. If you read up on the early days of aviation you will also find that pilots were often their own mechanics, fixing the planes at stops and what not. In a way its not that we don't need the position it's that the task has been offloaded elsewhere.

Surely you mean GPS reciever and not transceiver?

My mistake you are correct, there is no transmission involved.

The basic job of a navigation pilot is to mark all the landmarks on his map and to verify whether the plane is going in the right direction.There is a high probability that the plane might miss a landmark and create a confusion among the pilot and the navigator. Now we have GPS,which is more accurate and easier for the pilot.You get every piece of information there is.


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What is the purpose of the string taped to the front of the canopy on gliders?
What is the purpose of the piece of string taped to a glider canopy? Is it effectively a turn coordinator, but then what advantages would it have over a gauge such as the ones fitted in powered aircraft?

Yes, the yaw string on the canopy of a glider is there to show you whether or not you are coordinated, especially during turns.  It indicates the relative wind, so will be straight unless the glider is slipping or skidding, in which case it will swing to the left or right.
For those not familiar with them, here is an image that shows one:

According to Wikipedia, they are also used on other aircraft (even jet fighters and the U-2!) although they are used in almost every glider.
Now, the advantages that it has over an actual instrument installed in the panel are numerous:

It is directly in your line of sight while looking outside, so you don't have to look at the instrument panel to see if you are yawing.
It is more sensitive
It reacts faster to changes
It is cheaper
It is lighter
It doesn't require electricity (or even an electrical system on the glider)
It costs far less to maintain
It is easy to recognize when it fails (it's either there or not)

@Lnafziger The strings on the wings serve the same purpose? How are they different than the one on canopy?

@Farhan I haven't seen one on a wing.  Do you have a picture?

@Lnafziger A video is worth a thousand pictures! Here you go.

@Lnafziger Picture. I have seen strings in the Cessna course.

@Farhan Ahhh, they use those during certification flights so that they can observe the airflow over the wing.  You won't normally see those!

Here's one on an F-14D!  You can see it just above the radome. http://www.navy.mil/management/photodb/photos/060922-N-1129M-017.jpg

It doubles as a weather indicator:  If String is wet, its raining.  If String is White, it is snowing.  If String is stiff, it is freezing. If String is Dark, it is night-time. :)

@abelenky if string is smoking, you're on fire.

The slip-skid ball is a level with ball instead of bubble. It does not require electricity, is cheap, simple and reacts fast. It measures something different, but glider can't have asymmetric thrust, so the quantities should be well correlated.

@JanHudec Well, I'm not sure that I would call it cheap or simple when comparing it to a small length of string.  :)


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What are these autonomous, shape-shifting indoor neutral-density craft I read about?
This may be on the edge of off-topic. I am trying to read more about a technology where generally autonomous (though could be sent commands), neutral-buoyancy craft were navigating indoors, in large spaces. The image I have in mind - vaguely - is of huge, transparent whales swimming around in the air in a giant atrium, never colliding with each other or the walls or floor or ceiling.
I'm trying to read more about how the sensing was done and the algorithms used, and how it was made to be so light weight, and what kind of propulsion they used that was also so light weight.
I seem to remember that they were possibly even changing shape in order to change direction.
But all I have is this image in mind from roughly 3 to 6 years ago, and so far I can't find search terms that give me anything even close. There were several articles and videos about this at the time.

Do you mean the Festo Air Ray?

Ah! @Andy we're getting very close! They were shaped more like the Festo AquaPenguin except of course they worked in air like the Air Ray. But these have the mechanical "swimming" articulation I remember. That's fantastic! I can start reading about this now. If you know of a more in-depth engineering doc, that would be great too.

@mins - that's a good reference too, but they appear to have conventional propellors (fans), and look like they are not exactly balanced neutral buoyancy. Thanks for the links!

Andy - there is a Festo AirPenguin! If you search that, you'll see many images. That's exactly what I am thinking of. If you can post that as an answer, and possibly add a pic or a YouTube link, I can accept it. Bingo!  https://www.festo.com/cms/en_corp/9780.htm and also https://www.youtube.com/watch?v=jPGgl5VH5go

Those are friggin' cool!

The comment by @Andy suggesting they were Festo Air Rays almost nailed it.
They were Festo Air Penguins!

They can be seen maneuvering in these videos:

The article http://www.robaid.com/bionics/airpenguin-flying-robot-penguins-whats-next.htm describes the mechanism:

A strut to which the two wings are attached passes through the helium-filled balloon. This new type of wing design can produce either forward or reverse thrust. Each wing is controlled by two actuators: a flapping actuator for the up-and-down movement of the wings, and a further unit that displaces the wing strut to alter the pressure point of the wings. There is also a central rotational actuator for the two flapping wings that direct their thrust upwards or downwards, thus making the AirPenguins rise or descend.
The wing strut, which is supported at the pivot point of the torso, can be moved either towards the front or rear edge of the wing. Displacing the strut towards the front, for example, causes the wing’s pressure point to migrate forwards. The air-stream pressure bends the cross-section of the wing in such a way as to produce a profile that generates forward thrust. If the wing strut is moved towards the rear edge of the wing, the pressure point is likewise moved to the rear and the AirPenguin flies backwards. With this design a self-regulating, wing pressure-controlled, passively twisting adaptive wing has been realized for the first time.
The AirPenguins are also equipped with complex navigation and communication facilities that allow them to explore the space they float in on their own initiative, either autonomously or in accordance with fixed rules.

While they can be autonomous, and fly in formation, they can also maintain a "ground link."

By means of XBee, based on ZigBee, large volumes of data can be transmitted between the penguins and the transmitting stations by 2.4-GHz band radio. The penguins recognize each other on the basis of their distances to the transmitting stations. The rapid, precise control allows the AirPenguins to fly in a group without colliding, while also mastering height control and positional stability. As an alternative, they can act synchronously as a group. A comprehensive central surveillance system provides security in case of sensor failure. When AirPenguins are on low energy supply it prompts the penguins to return to the charging station.
The ongoing development of sensor and control technology is thus also being promoted along the road to decentralized, autonomously self-controlling and self-organizing systems thanks to the inspiration from nature.

While searching for information I found this even more advanced, articulated light (but not neutral buoyant) craft, the BoiniCopter, also http://www.robaid.com/bionics/festo-bionicopter-robot-dragonfly.htm which states a mass of 175 grams and wingspan of 70cm :

note: I have no affiliation with these sites or companies. This answer is a bit long I know, but the material is a bit unique. I felt that in this particular case the extra background may be helpful.

Great post! One quibble: the bionicopter dragonfly is heavier than air.

@RoboKaren Thanks for that! - I was typing on auto-pilot. The robaid.com link for the BioniCopter says it is 175 grams, which means it would have to have roughly 160 litres of Helium, which would be like a 27 inch diameter sphere. I'll correct the text,


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Runway landing with a front skid instead of wheel - how does steering happen? (Dreamchaser)
The Spaceflight Now article Sierra Nevada’s Dream Chaser performs critical glide test flight shows the Dreamchaser performing a runway landing using a front skid and two wheels.
The skid extends backwards from the point where it connects to the aircraft. I am guessing that it can turn for steering once in contact with the runway during landing. Are front skids an established method for runway landings? Can they be used to steer by rotating? If so, is there a simple way to understand how this works - how would rotating the skid actually change the direction of the aircraft; how does the frictional force from forward motion produce a lateral force that changes the direction of the aircraft's motion?
Screen shot cropped from Dream Chaser Spacecraft Extended Free Flight

Images (cropped from originals) from here.

Like many aircraft that dont have a steerable nose wheel they most likely use differential braking. Here is a pretty interesting paper that covers some of the testing they did on the airframe. Steering is not really a large concern for the Dream Chaser (or the space shuttle for that matter) as it has no power to maneuver on the ground anyway. Generally speaking it will be towed off the runway.
According to this source SNC notes similar reasoning to the X-15, simplicity as well as concerns around tires in a space environment (which is interesting seeing as the space shuttle had them).

Speaking of the rationale for opting to use a skid strip, SNC noted it
  is a simple, light, safe option. They also added that there had been
  some issues with the analysis of the performance of tires in the space
  environment and this eliminates one of the tires – with the other two
  tires easier to control than the nose wheel if there’s a problem with
  one of them.
SNC noted they do not expect any crosswind limitations from the skid.

Although here is a fairly interesting NASA press release on testing the "nose gear" for the Dream Chase implying there may have at one point been gear there.

In fact, the Space Shuttle didn't originally have nosewheel steering. It was added after brake damage incidents.

Doesn't the term "landing gear" include skids as well as wheels? Maybe this skid is the "nose landing gear" mentioned in the press release.

Seems to me the Shuttle orbiters came back with one or more flat tires on at least a couple occasions -- but with 95 tons landing at near 100 m/s, the tires aren't that functional anyway.

"Steering is not really a large concern for the Dream Chaser (or the space shuttle for that matter) as it has no power to maneuver on the ground anyway. Generally speaking it will be towed off the runway." -- isn't the deceleration time after touchdown long enough that there are indeed many circumstances where steering might be an issue?


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Which international routes might save fuel and time by using a potential newly "deemed safe" North Korean air space?
The Reuters News item Exclusive: U.S. blocks North Korean air traffic revival ahead of Trump-Kim summit - sources says:

If the space was deemed safe, international airlines could save fuel and time on some routes between Asia and Europe and North America, and North Korea could begin reviving its own commercial aviation industry.

Question: Which international routes in particular, flown by international airlines could save fuel and time on some routes between Asia and Europe and North America by using newly "deemed safe" North Korean air space?

One potential would be KE74

I think the biggest one would be Tokyo - Beijing, and more broadly, anything in Japan north of Osaka to Beijing.

The current airway turns does a dogleg up to right above Incheon airport, (you can literally see the airport and Seoul).
You can play with http://www.gcmap.com to see great circle maps, which are a good approximation of the most efficient route.